The introduction of high activity Ziegler-Natta catalyst systems has led to the development of new polymerization processes based on gas phase reactors as disclosed in U.S. Pat. No. 4,482,687, issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefin polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact, new and better products have been synthesized using single- and multiple-, or staged-, gas phase reactor systems because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
Conventional gas phase fluidized bed reactors used in polymerizing alpha-olefins have a cylindrical shaped fluidized bed portion and an enlarged, tapered-conical entrainment disengaging section, sometimes referred to as the expanded section. Solid particulates are projected upward into the expanded section through the bursting of rising gas bubbles at the surface of the fluidized bed, and most of these particulates are typically returned to the fluidized bed by gravity as their velocity dissipates in the lower gas velocities of the expanded section. A small quantity of fine powder, or fines, is elutriated out of the projected particulates and does not return directly to the fluid bed by gravity. These fines are either conveyed upward by the cycle gas to the reactor gas outlet, or settle on surfaces of the expanded section through gravity or through particle attraction forces such as electrostatic attraction.
Disengaged fines that settle on the expanded section surfaces are known to accumulate as layers of fines under certain conditions. Settled fines are generally reactive and continue to polymerize in place at a rate related to the concentration of active catalyst contained in the fines. Such layers typically build to sufficient thicknesses in a short period of time that the forces holding them in place are overcome by gravity and the layers then slide harmlessly back into the fluid bed. Larger particulates from the fluid bed may also be projected onto the layers of fines, especially at lower elevations near the bed surface, causing all or part of the layer to be released and to then re-enter the bed through gravity. The cycle of fines buildup and return to the bed occurs repetitively in normal operation.
Under certain conditions, depending on factors such as electrostatic forces on the particles, the time cycle for return of the fines layers to the fluid bed becomes longer than normal. Longer cycle times are highly undesirable, since continued polymerization within the settled fines continues at temperatures higher or lower than the controlled bed temperature. Stagnant layers of fines are self-insulating, and, therefore, heat accumulation from continued polymerization within the layer can lead to temperatures above the sintering or melting point resulting in formation of molten sheets of resin, known as sheeting. Sheeting is the formation and adherence of fused catalyst and resin particles on the walls of a reactor, particularly in the expanded section. When the sheets are disturbed or become heavy, they can fall off the walls and plug the product discharge system or clog the distributor plate. Sheets and retained fines from the expanded section also contribute to product quality problems by increasing the level of non-specification contamination such as high molecular weight gels in end-use products such as plastic containers and films. Sheeting and fines accumulations are collectively referred to as solid particle build-up.
Conversely, expanded section fines layers that are relatively inactive are cooled by the reactor wall to temperatures below reactor temperature, resulting in much higher molecular weights and other product properties that are different from the average of the fluidized bed. These fines increase the level of non-specification contamination in end-use products, causing undesirable irregularities such as high molecular weight gels, and may be sufficient to cause downgrading of the resin product to lower quality grades of significantly reduced value.
The enlarged entrainment disengaging section is employed to minimize the quantity of fine powder, or fines, entrained by the cycle gas into the gas cycle system. Fines exiting the reactor with the cycle gas are generally conveyed through the gas cycle system before re-entering the fluidized bed at the bottom, but a smaller portion of fines adheres to surfaces of the gas cycle system. Such fines promote undesirable polymer growth and fouling of surfaces in the cycle piping, cycle cooler, compressor, lower reactor head, and distributor plate resulting in undesirable reactor shutdowns for system cleaning. Adhered particles in the cycle system continue to polymerize over time under process conditions different from the fluid bed, forming polymer of significantly different properties, such as molecular weight, density, and molecular weight distribution, from that of the fluid bed. Some particles are eventually released from the cycle system surfaces and are conveyed by the cycle gas back into the fluid bed. Such particles contaminate and adversely affect properties of the polymer product, such as by increasing the gel level in end-use products such as plastic containers and films.
Conventionally, to prevent solid particle build-up from affecting these and other parts of the reactor system, as well as the final polymer product, the reactors are shutdown periodically and the walls are cleaned to remove particle buildup. When a reactor is down for cleaning, it is typically hydro-blasted, sand-blasted, or shell-blasted using high pressure jets to remove sheets and fines build-up. Since water and oxygen introduced during the blasting process are strong catalyst poisons, the reactor must be purged to remove these poisons and the reactor must be dried. This process is both time consuming and costly. As a result, significant savings can be obtained with the prevention of a single shutdown.
It is also conventional practice to maintain the level of the fluidized bed a few feet below the neck of the expanded section to avoid the accumulation of fines in the expanded section. Thus, the volume of the fluidized bed, and, therefore, the amount of polymer in the reactor is conventionally controlled at a fixed level to avoid the undesirable effects of solid particle build-up in the expanded section.
Measures allowing the fluidized bed volume to be temporarily reduced without solid particle build-up are highly desirable, since the amount of lower value off-grade product generated during reactor start-up and grade changes may be reduced. Typical grade transitions require one to three bed turnovers depending on the specific product grades. By temporarily lowering the fluidized bed volume during reactor start-up and grade changes, the quantity of polymer embodied in a given number of bed turnovers may be substantially reduced in direct proportion to the reduction of bed volume.
Lowering the fluidized bed level and at the same time maintaining a constant high production rate will increase the bed volume turnover rate and directly reduce the polymer residence time. During the operation of the gas phase fluidized bed polymerization reactor system, there are times when it is highly desirable to adjust the powder inventory and/or solids residence time. Catalyst productivity and polymerization rate are affected by the residence time of the resin and catalyst in the reactor. Control of catalyst productivity and polymerization rate by adjusting residence time is a desirable method for controlling reactors that are operated in sequence (i.e., staged reactors) to produce products such as bimodal polymers or copolymers. In these types of polymerization processes, control of the proportion of polymer made in each reactor plays a key role in determining the properties and consistency of the final product. Thus, the freedom to apply such measures to temporarily reduce the reactor bed volume without risk of solid particle build-up is desirable and commercially important for the production of certain products, such as bimodal polymers or copolymers produced in staged reactors operated in sequence.
It would be most desirable to improve reactor operation and product quality by reducing sheeting and the accumulation of fines in the reactor expanded section and gas cycle system.